The present invention relates to an objective lens used for at least recording or reproducing high-density optical recording medium such as the so-called optical disks. In particular, the present invention relates to an objective lens for recording and reproducing high-density optical information using a short wavelength light source and manufacturing method therefor.
CD (compact disks) that have been widely used as optical recording medium are employed mainly in a numerical aperture range of 0.45–0.5. Furthermore, DVD (digital versatile disks) conduct optical information recording in a numerical aperture (NA) range of 0.6–0.65 by using a light source with a wavelength of 650–780 nm.
However, as a need for increased capacity has been generated, a demand arose for high-density optical information recording medium capable of recording with a higher recording density and optical systems for such recording and reproduction. Accordingly a demand for higher NA was created in the field of objective lenses used for such applications, that is, in the optical systems for recording and reproducing with such high-density optical information recording media.
Laid open Japanese Patent Application No. 2001-324673 describes an objective lens for use in optical systems for recording and reproducing high-density optical information recording media, this lens being advantageously suitable for high-density recording and reproduction devices with a numerical aperture NA of 0.75 or larger and a light source wavelength of about 400 nm.
Further, this prior art document describes an aspherical single-lens objective lens with an axial length thickness d1 and a focal distance f satisfying the following condition1.1≦d1/f≦3,or a lens in which an Abbe number νd, refractive index n at a usage wavelength, and a paraxial curvature radius r1 at the light source side satisfy the following conditions:
1.2 ≦ d1/f ≦ 2.3f/νd ≦ 0.0601.40 ≦ n1.40 ≦ n < 1.850.40 ≦ r1/(n × f) ≦ 0.70.
Further, Laid-open Japanese Patent Application No. 2002-156579 suggested a biaspherical lens with a numerical aperture NA of 0.7 or more and a central thickness of the lens larger than the focal distance as an objective lens suitable for such transition to increased density.
In large-capacity optical disks of the next generation, a wavelength close to 400 nm which is shorter than that of the conventional technology has to be used as a light source wavelength for increasing the recording density. In this case, for example, a blue-violet semiconductor laser is needed as a light source and an aspherical lens with a numerical aperture NA of 0.8 or larger is required as an objective lens.
In single-lens objective lenses for optical disks, a certain correlation exists between the off-axis aberration, eccentricity tolerance, or chromatic aberration and the lens thickness. In the conventional objective lenses for CD or DVD, the range of lens thickness which is allowed by the required aberration is comparatively wide owing to the range of values of the numerical aperture NA thereof. This is why it was possible to conduct lens design and determine lens thickness based exclusively on physical limitations such as working distance and lens weight. However, in single-lens objective lenses for large-capacity optical disks, the aberration tends to increase due to higher NA. A range of lens thickness that can be selected to obtain sufficient optical performance, that is, to reduce the aberration to below the prescribed range, is limited. As a result, in single-lens objective lenses for higher NA, the effective lens thickness becomes larger with respect to a focal distance than that of the objective lenses for conventional CD or DVD.
If the lens thickness thus increases, the volume of the lens material increases accordingly. Thus, the radius r of the premolded optical material which is used as the lens material, for example, the radius of a spherical preform, becomes larger. The resultant problem is that if the radius r exceeds the paraxial curvature radius R (almost equal to paraxial curvature radius R of the lens that will be molded) of the molding surface in the mold, the desired lens surface accuracy is difficult to obtain.
For example, with such lenses, if the curvature radius R of the molding surface of the mold is less than the curvature radius r of the molding material that was premolded prior to molding, that is, the preform (for example, a glass preform composed of glass), when a lens having a convex surface is molded by a mold press molding process, then a preform with a curvature radius larger than that of the concave molding surface of the mold will be disposed at this molding surface during press molding.
If press molding is to be conducted in this state, then an atmosphere gas such as air or the prescribed gas will be confined between the molding surface and the preform. If molding is then conducted without releasing this gas, the described lens surface accuracy will not be attained. For example, a recess will be formed in the molded lens.
With the foregoing in view, it is an object of the present invention to provide an objective lens which makes it possible to inhibit the aberration in an objective lens for optical information recording and reproduction, which is composed of a molded aspherical single lens, while attaining at the same time good lens productivity, and also to ensure excellent optical properties and to obtain a high production efficiency in a mold processing step conducted to process the molding surface of the mold and in press molding of a lens manufacture step, and also to provide a method for the manufacture of such a lens.
Thus, it is an object of the present invention to provide a method for the manufacture of an objective lens by which, in particular, molding is effectively prevented from being conducted in a state in which a gas is confined between the preform and the mold and a lens with a high surface accuracy can be molded.
Further, it is an object of the present invention to provide a method for the manufacture of an objective lens by which, in particular, productivity can be further increased and the off-axis aberration and chromatic aberration can be further improved.
It is another object of the present invention to provide a method for the manufacture of an objective lens which is advantageously suitable in particular for a lens of an infinite system with respect to at least a standard wavelength.
It is another object of the present invention to provide a method for the manufacture of an objective lens by which, in particular, a working distance is ensured, and the off-axis view angle aberration, chromatic aberration, and also the increase in the lens weight can be effectively inhibited.
It is another object of the present invention to provide a method for the manufacture of an objective lens by which, in particular, the process of assembling an optical pickup using such a lens can be further facilitated.
It is object of the present invention to provide a method for the manufacture of an objective lens by which, in particular, a high refractive power can be obtained without drastically increasing the curvature of the lens curved surface, precision processing of the molding surface of the mold can be conducted comparatively easily, the chromatic aberration can be effectively reduced, and a mold material can be easily selected.
It is another object of the present invention to provide a lens with a high surface accuracy, such that molding is effectively prevented from being conducted in a state in which a gas is confined between the preform and the mold.